Steps to reproduce:
1. Have Tellah, Great Sage and an effect that doubles his trigger like Harmonic Prodigy on the battlefield.
2. Spend 8 or more mana to cast a noncreature spell.
3. Tellah will trigger twice, the first one will do all 3 effects but when the second ability resolves you only draw 2 cards and make your hero token.

The ruling on gather for him state: "If Tellah is no longer on the battlefield when its ability resolves and eight or more mana was spent to cast the spell that caused its ability to trigger, the ability will still deal damage to each opponent."

and I confirmed this with the judge chat to make sure.